The heart is a muscular organ comprising multiple chambers that operate in concert to circulate blood throughout the body's circulatory system. As shown in FIG. 1, the heart 100 includes a right-side portion or pump 102 and a left-side portion or pump 104. The right-side portion 102 includes a right atrium 106 and a right ventricle 108. Similarly, the left-side portion 104 includes a left atrium 110 and a left ventricle 112. Oxygen-depleted blood returning to the heart 100 from the body collects in the right atrium 106. When the right atrium 106 fills, the oxygen-depleted blood passes into the right ventricle 108 where it can be pumped to the lungs (not shown) via the pulmonary arteries 117. Within the lungs, waste products (e.g., carbon dioxide) are removed from the blood and expelled from the body and oxygen is transferred to the blood. Oxygen-rich blood returning to the heart 100 from the lungs via the pulmonary veins (not shown) collects in the left atrium 110. The circuit between the right-side portion 102, the lungs, and the left atrium 110 is generally referred to as the pulmonary circulation. After the left atrium 110 fills, the oxygen-rich blood passes into the left ventricle 112 where it can be pumped throughout the entire body. In so doing, the heart 100 is able to supply oxygen to the body and facilitate the removal of waste products from the body.
To circulate blood throughout the body's circulatory system as described above, a beating heart performs a cardiac cycle that includes a systolic phase and a diastolic phase. During the systolic phase (e.g., systole), the ventricular muscle cells of the right and left ventricles 108, 112 contract to pump blood through the pulmonary circulation and throughout the body, respectively. Conversely, during the diastolic phase (e.g., diastole), the ventricular muscle cells of the right and left ventricles 108, 112 relax, during which the right and left atriums 106, 110 contract to force blood into the right and left ventricles 108, 112, respectively. Typically, the cardiac cycle occurs at a frequency between 60 and 100 cycles per minute and can vary depending on physical exertion and/or emotional stimuli, such as, pain or anger.
The contractions of the muscular walls of each chamber of the heart 100 are controlled by a complex conduction system that propagates electrical signals to the heart muscle tissue to effectuate the atrial and ventricular contractions necessary to circulate the blood. As shown in FIG. 2, the complex conduction system includes an atrial node 120 (e.g., the sinoatrial node) and a ventricular node 122 (e.g., the atrioventricular node). The sinoatrial node 120 initiates an electrical impulse that spreads through the muscle tissues of the right and left atriums 106, 110 and the atrioventricular node 122. As a result, the right and left atriums 106, 110 contract to pump blood into the right and left ventricles 108, 112 as discussed above.
At the atrioventricular node 122, the electrical signal is momentarily delayed before propagating through the right and left ventricles 108, 112. Within the right and left ventricles 108, 112, the conduction system includes right and left bundle branches 126, 128 that extend from the atrioventricular node 122 via the Bundle of His 124. The electrical impulse spreads through the muscle tissues of the right and left ventricles 108, 112 via the right and left bundle branches 126, 128, respectively. As a result, the right and left ventricles 108, 112 contract to pump blood throughout the body as discussed above.
Normally, the muscular walls of each chamber of the heart 100 contract synchronously in a precise sequence to efficiently circulate the blood as described above. In particular, both the right and left atriums 106, 110 contract (e.g., atrial contractions) and relax synchronously. Shortly after the atrial contractions, both the right and left ventricles 108, 112 contract (e.g., ventricular contractions) and relax synchronously. Several disorders or arrhythmias of the heart can prevent the heart from operating normally, such as, blockage of the conduction system, heart disease (e.g., coronary artery disease), abnormal heart valve function, or heart failure.
Blockage in the conduction system can cause a slight or severe delay in the electrical impulses propagating through the atrioventricular node 122, causing inadequate ventricular relaxation and filling. In situations where the blockage is in the ventricles (e.g., the right and left bundle branches 126, 128), the right and/or left ventricles 108, 112 can only be excited through slow muscle tissue conduction. As a result, the muscular walls of the affected ventricle (108 and/or 112) do not contract synchronously (e.g., asynchronous contraction), thereby, reducing the overall effectiveness of the heart 100 to pump oxygen-rich blood throughout the body.
Various medical procedures have been developed to address these and other heart disorders. In particular, cardiac resynchronization therapy (“CRT”) can be used to improve the conduction pattern and sequence of the heart 100. CRT involves the use of an artificial electrical stimulator that is surgically implanted within the patient's body. Leads from the stimulator can be affixed at a desired location within the heart 100 to effectuate synchronous atrial and/or ventricular contractions. Typically, the location of the leads (e.g., stimulation site) is selected based upon the severity and/or location of the blockage. Electrical stimulation signals can be delivered to resynchronize the heart, thereby, improving cardiac performance.
In conventional CRT systems, establishing synchronization of ventricular walls involves measuring intrinsic electrical signals traveling through the ventricles and then applying an educated guess to time application of stimulation signals that are intended to provide the resynchronization. The educated guess employed by the CRT device is generally based upon empirical data of heart failure patients' responses to application of electrical signals with varying timing, amplitude, and/or location. However, the success of the stimulation signals in terms of the mechanical response of the ventricles can only be known with certainty by directly measuring the mechanical response occurring due to application of the stimulation signal.
Observing the mechanical response is useful when optimizing CRT because there may be no known correlation between the electrical activity and mechanical response of a particular patient's heart. Improving the heart's mechanical response is ultimately the goal of CRT, and therefore controlling the mechanical response by measuring only electrical activity is not ideal. An additional drawback to conventional CRT methods of optimization includes a requirement that the CRT stimulation be turned off so that the implanted electrodes can sense rather than stimulate. This causes the measured electrical activity used to optimize the CRT device to be even further removed from the heart's mechanical response to stimulation from CRT.
Direct mechanical measurements of ventricular wall synchronization allow the mechanical effects of manipulating the electrical activity to be observed. Direct mechanical measurements can be performed by ultrasound techniques or other similarly cumbersome and time-consuming methods that are available only when the patient is visiting a health care provider. These relatively infrequent follow-up visits are inadequate for constantly optimizing the CRT therapy because the mechanical response of the heart may change as the patient's physical condition changes over time. Thus, the patient's CRT device may become inefficient during the time between visits. Additionally, these direct mechanical measurements are performed by systems distinct from the patient's CRT device and do not automatically optimize CRT based on the mechanical response but require intervention by a physician.
Therefore, there is a need for CRT methods and devices that can constantly and/or automatically optimize CRT for a patient based on mechanical response of the patient's ventricles.